Combined neutron and gamma-ray detector and coincidence test method

ABSTRACT

A method for detecting both gamma-ray events and neutron events with a common detector, where the detector includes a layer of semiconductor material adjacent one side of a glass plate and a Gd layer on an opposite side of the glass plate, between the glass plate and a layer of silicon PIN material to form an assembly that is bounded by electrodes, including a semiconductor anode on one side of the semiconductor layer, a cathode connected to the glass plate, and a Si PIN anode on a side of the Si PIN layer opposite the semiconductor anode. The method includes the steps of: (1) monitoring the electrical signal at each of the semiconductor anode and the Si PIN anode, and (2) comparing signals from the semiconductor anode and the SI PIN anode to differentiate between gamma-ray events and neutron events based on predetermined criteria.

FIELD OF THE INVENTION

This invention concerns a detector for both neutrons and gamma rays anda corresponding method for improving the detection of neutrons and gammarays in a common detector.

BACKGROUND

Radiation detectors have many important uses in nuclear energy, physicsresearch, materials science, and radiation safety, among others. Twotypes of radiation often of interest are neutrons and gamma rays.

One way to detect these types of radiation uses a scintillator materialcalled CLYC (which is Cs2LiYCl6:Ce3+), typically in the form of acrystal. Like other scintillators, a CLYC crystal produces a flash oflight when capturing a gamma ray. The flash of light can be turned intoan electrical signal for further analysis. A CLYC crystal also can beused to capture neutrons through a nuclear reaction with lithium (Li)atoms in the crystal, and also produces a flash of light due to theenergetic particles from the neutron-lithium reaction. Unfortunately,these crystals can be difficult to grow and thus are quite expensive,and it can be difficult to distinguish the flashes of light due toneutrons from the flashes of light due to gamma rays.

Another way to detect these types of radiation relies on the capture ofneutrons by cadmium (Cd) in crystals of cadmium-zinc-telluride (CdZnTe)(often abbreviated CZT). CZT also is used in detectors for gamma-rayradiation. The neutron-cadmium reaction produces gamma rays that can bedetected by pulses of electrons from the CZT, but the sensitivity is lowand it is difficult to distinguish whether the pulse of electrons wascaused by a neutron or a gamma ray.

SUMMARY

The present invention provides a combined neutron and gamma-ray detectorand method that is sensitive to both neutrons and gamma-rays in the samedetector, improves the ability to distinguish between the two kinds ofradiation, is compact, requires relatively little power, and isrelatively inexpensive compared to current radiation detection devicesand methods.

More specifically, the present invention provides a method for detectingboth gamma-ray events and neutron events with a common detector thatincludes a layer of semiconductor material adjacent a glass plate, agadolinium (Gd) converter layer adjacent an opposite side of the glassplate, and a layer of silicon PIN material in contact with the Gdconverter layer on an opposite side of the glass plate to form anassembly that is bounded by electrodes, including a semiconductor anodeon one side of the semiconductor layer, a cathode connected to the glassplate, and a Si PIN anode on a side of the Si PIN layer opposite thesemiconductor anode. The method includes the steps of: (1) monitoringthe electrical signal at each of the semiconductor anode and the Si PINanode, and (2) comparing signals from the semiconductor anode and the SIPIN anode to differentiate between gamma-ray events and neutron eventsbased on predetermined criteria.

The method may further include one or more of the steps of establishingan electric field within the semiconductor layer and the Si PIN layer,and providing a common gamma-ray and neutron detector with a controllerconnected to the first and second anodes and the cathode, the controllerincluding a processor and a memory.

The present invention also provides a detector for both gamma-rays andneutrons that includes (a) a semiconductor layer including asemiconductor material suitable for capturing gamma-rays, (b) a glassplate in contact with the semiconductor layer, (c) a gadolonium (Gd)converter layer in contact with the glass plate opposite thesemiconductor layer, (d) a layer of silicon PIN(p-type/intrinsic/n-type) material of suitable thickness for detectingelectrons emitted from neutrons captured by the Gd converter layer incontact with the silicon PIN layer opposite the glass plate, (e) acathode contact in electrical contact with the glass plate, (f) a firstanode contact in contact with the semiconductor layer, (g) a secondanode contact in contact with the silicon PIN layer, and (h) a processorin electric contact with the first and second anode contacts. Theprocessor is configured to cooperate with the anode contacts and thecathode contacts to establish electric fields across the semiconductorlayer and the Si PIN layer, and is configured to differentiate betweensignals generated by a neutron event and signals generated by agamma-ray event.

The semiconductor material may include any of cadmium-zinc-telluride(CdZnTe), high-resistivity gallium arsenide (GaAs), and high puritygermanium (HPGe).

The detector may further include a printed circuit board (PCB) inelectrical contact with respective ones of the first and second anodecontacts to connect the first and second anode contacts to respectivecontacts in respective processors. A plurality of adjacent spaced-apartdetectors may share a common PCB.

The first and second anode contacts may be divided into an array ofpixels. The semiconductor layer may include cadmium-zinc-telluride(CdZnTe) and may be separated into pixels aligned with pixels in the SiPIN layer.

The semiconductor layer may have a thickness of 0.35 cm, the glass platemay have a thickness of at least 300 μm, the Gd layer may have athickness of 5 μm, and the Si Pin layer may have a thickness of 280 μm.Alternatively, the semiconductor layer may have a thickness of 1.0 cm or1.5 cm or 2.0 cm.

The present invention further provides a controller for use with acommon detector for both gamma-rays and neutrons, where the detectorincludes a semiconductor layer including a semiconductor materialsuitable for capturing gamma-rays, a glass plate in contact with thesemiconductor layer, a gadolonium (Gd) converter layer in contact withthe glass plate opposite the semiconductor layer, a layer of silicon PIN(p-type/intrinsic/n-type) material of suitable thickness for detectingelectrons emitted from neutrons captured by the Gd converter layer incontact with the silicon PIN layer, a cathode contact in electricalcontact with the glass plate, a first anode contact in contact with thesemiconductor layer, and a second anode contact in contact with thesilicon PIN layer. The controller includes a processor configured tocooperate with the first anode, the second anode and the cathode toestablish an electric field across the semiconductor layer and thesilicon PIN layer. The processor is configured to differentiate betweensignals generated by a neutron event and signals generated by agamma-ray event as a function of coincidence testing of signals receivedfrom each of the first and second anodes.

The present invention also provides a detector for both gamma rays andneutrons that includes (a) means for capturing gamma-rays that producesan electron, (b) means for capturing neutrons that produces an electron,(c) means for separating electrons generated by the means for capturinggamma rays from the electrons generated by the means for capturingneutrons, (d) means for applying an electric field across the means forcapturing neutrons and the means for capturing gamma rays, and (e) meansfor differentiating between signals generated by a neutron event andsignals generated by a gamma-ray event.

The means for capturing gamma-rays may include a semiconductor layerhaving a semiconductor material suitable for capturing gamma-rays.

The means for capturing gamma-rays may include a layer of crystallizedcadmium-zinc-telluride (CdZnTe) (CZT).

The means for capturing neutrons that produces an electron may include agadolonium (Gd) converter layer for capturing neutrons and producing anelectron and a layer of silicon PIN (p-type/intrinsic/n-type) materialof suitable thickness in contact with the Gd converter layer fordetecting the electron The means for separating electrons generated bythe means for capturing gamma rays from electrons generated by the meansfor capturing neutrons may include a glass plate.

The (d) means for applying an electric field across the means forcapturing neutrons and the means for capturing gamma rays may include(1) a cathode contact in electrical contact with the separating means,(2) a first anode contact in contact with the gamma-ray capturing means,(3) a second anode contact in contact with the neutron capturing means,and (4) a processor in electric contact with the first and second anodecontacts. The processor may be configured to cooperate with the firstand second anode contacts and the cathode contacts to establish anelectric field across the means for capturing neutrons and the means forcapturing gamma rays.

The means for differentiating between signals generated by a neutronevent and signals generated by a gamma-ray event may include a processorin electric contact with the means for establishing an electric fieldacross the means for capturing neutrons and the means for capturinggamma rays. The processor is configured to differentiate between signalsgenerated by a neutron event and signals generated by a gamma-ray event.

The foregoing and other features of the invention are hereinafter fullydescribed and particularly pointed out in the claims, the followingdescription and the annexed drawings setting forth in detail one or moreillustrative embodiments of the invention. These embodiments, however,are but a few of the various ways in which the principles of theinvention can be employed. Other objects, advantages and features of theinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of a combined neutron and gamma-raydetector provided by the invention.

FIG. 2 is a top view of the detector of FIG. 1.

FIGS. 3A and 3B are graphical representations of gamma ray energyresolution in the detector.

FIG. 4 is a graphical representation of the energy spectrum of internalconversion electrons due to neutrons received in the detector.

FIG. 5 is a schematic cross-section of a portion of the detectorillustrating a neutron event.

DETAILED DESCRIPTION

As noted above, the present invention provides a combined neutron andgamma-ray detector and method that employs a coincidence test for pulsesin two different detection materials to detect both thermal ornear-thermal neutrons and gamma rays.

The detector combines two different detector materials in a commonstructure, one of which provides a means for capturing gamma-rays thatproduces an electron, and the other of which provides a means forcapturing neutrons that produces an electron. The detector furtherincludes means for separating the electrons generated by the respectivemeans for capturing gamma-rays and the means for capturing neutrons. Thedetector also includes means for establishing an electric field acrosseach of the means for capturing gamma-rays and the means for capturingneutrons. And the detector includes means for differentiating betweensignals generated by a neutron event and signals generated by agamma-ray event.

An exemplary means for capturing gamma-rays includes a layer ofsemiconductor material configured to capture gamma rays, such asCadmium-Zinc-Telluride (CdZnTe) (referred to as CZT), which is availablefrom eV Microelectronics of Saxonburg, Pa., U.S.A., for example. Anexemplary means for capturing neutrons includes a layer of a gadolonium(Gd), referred to as a Gd converter layer. The Gd converter layerproduces an electron when a neutron is captured, and the electron isthen detected in a layer of silicon PIN (p-type/intrinsic/n-type)material of suitable thickness. The detector also includes a controllerconnected to the layers of detector materials that provides a means forestablishing electric fields across the CZT layer and the Si PIN layer,and uses a coincidence test to check for pulses occurring in the twomaterial layers and differentiates between signals generated todetermine whether a pulse is the result of a neutron or a gamma ray.

Turning now to the drawings in detail, and starting with FIG. 1, anexemplary detector 10 for detecting both gamma-rays and neutrons isshown. The detector 10 has a compact all-solid-state structure thatincludes a series of layered elements that form a common detectorassembly. Although the illustrated embodiment includes two detectorassemblies, the detector 10 could function with just one or anyreasonable number of detector assemblies. The detector assemblies areidentical, and reference to either a detector 10 or to a detectorassembly generally will be equivalent.

Each detector 10 includes a layer of semiconductor material 12 suitablefor capturing gamma-rays, such as a layer of Cadmium-Zinc-Telluride(CdZnTe) (CZT), aligned with a layer of silicon (Si) PIN (siliconp-type/intrinsic/n-type) 14 of the same area and having a suitablethickness for detecting electrons produced by neutrons, as furtherdescribed below. Although the illustrated embodiment will be describedwith reference to the CZT layer 12, other semiconductor materials may beused in place of CZT, including high-resistivity gallium arsenide (GaAs)and high purity germanium (HPGe). The CZT layer 12 is relatively thickcompared to the other layers. The CZT layer 12, because of itscomposition of relatively heavy atoms and thick configuration, issensitive to gamma rays, including those not connected with a neutroncapture, as well as those resulting from a neutron capture.

In contrast to the CZT layer 12, the Si PIN layer 14 is relativelyinsensitive to gamma rays because of its composition of relatively lightatoms and thin configuration. A glass plate 16 or other insulatingmaterial in contact with the CZT layer 12 isolates and separates the SiPIN layer 14 from the CZT layer 12. The glass plate 16 separating theCZT and Si PIN layers 12 and 14 generally is relatively thin incomparison but has sufficient thickness to provide means for separatingelectrons generated in the CZT layer 12 from electrons detected in theSi PIN layer 14. The glass plate 16 prevents electrons from travelingbetween the CZT layer 12 and the Si PIN layer 14.

The means for capturing neutrons includes a gadolinium (Gd) converterlayer (Gd layer 20). The Gd layer 20 is relatively thin and liesadjacent to the glass plate 16 separating the Gd layer 20 and the CZTlayer 12. Thus the Gd layer 20 is in contact with one side of the glassplate 16 opposite the CZT layer 12, between the Si Pin layer 14 and theglass plate 16. An exemplary Gd layer 20 is a 5-μm layer ofGadolinium-157 (¹⁵⁷Gd). The Gd layer 20 absorbs neutrons with highefficiency and consequently emits energetic electrons and gamma rays.The electrons naturally travel a much shorter path than the gamma rays.The Si PIN layer 14 is sensitive to the internal conversion electronscaused by a neutron capture in the adjacent Gd layer 20. The glass plate16 prevents the electrons in the Gd layer 20 from registering in the CZTlayer 12, which makes it easier to distinguish between events indicatingneutron capture compared to events indicating the presence of gammarays.

Each CZT/Si PIN assembly 24, including the foregoing layers 12, 14, 16,and 20, has an array of anode contacts that separate the CZT/Si PINassembly 24 into pixel areas or pixels 26, as shown in FIG. 2, forexample. The CZT/Si PIN assemblies 24 are sandwiched between a pair ofcontrollers 30 and 32 with logic instructions encoded as software orhard-wired. Each controller 30 and 32 typically has at least oneprocessor or Central Processing Unit (CPU), together with an associatedmemory for storing an operating system, application software, and datagenerated by events in the detector 10. The controller 30 or 32 mayinclude an analog-to-digital signal processor or converter, and may beconnected to input and output devices in a well known manner.

Multi-layer printed circuit boards (PCBs) 34 and 36 cooperate with theassociated controller 30 and 32 to connect each pixel area 26 of theCZT/Si PIN assembly 24 to a respective contact on the respectiveprocessor. The processor may include two or more silicon-basedapplication-specific integrated circuits (ASICs) that drive the CZT andSi PIN layers 12 and 14. To that end, first and second anode contactsare coupled to the processors or PCBs 34 and 36, in electric contactwith respective outer surfaces of the CZT layer 12 and the Si PIN layer14. Traces on the PCBs 34 and 36 connect the anodes to the correspondingunit cells of the ASICs or other processors. The processors also arecoupled to and in electric contact with cathode contacts, adjacent toand in electric contact with the glass plate 16. Thus the electrodes,the anodes and the cathodes, are connected to the processors inrespective controllers 30 and 32. The controller 30 and 32 controls theapplication of the electric field to the CZT/Si PIN assembly 24 throughthe anodes and the cathodes. The potential difference between the anodeand cathode contacts, controlled by the controller 30 and 32,establishes an electric field within each detection material (the CZTand Si PIN layers 12 and 14). The controller 30 and 32, and morespecifically the associated processors, also receives and analyzeselectrical signals received at the anodes and the cathodes usingcoincidence criteria to identify neutron events and gamma-ray events andto distinguish between them. Put another way, the processor of eachcontroller 30 and 32 is configured to differentiate between signalsgenerated by a neutron event and signals generated by a gamma-ray eventas a function of coincidence testing of signals received from each ofthe first and second anodes.

Although the detector 10 is not primarily intended for imaging,advantages of the pixel approach include:

1) the ability to turn off defective pixels 26, greatly increasing theyield of usable CZT material and reducing cost;

2) the use of multiple independent detectors 10 operating in parallelreduces dark current, capacitance, and pulse pile-up;

3) aligned CZT/Si PIN pixel pairs facilitate coincidence testing; and

4) the CZT/Si PIN assembly 24 topologically conforms to an existing ASICthat can perform depth correction calculations that improve gamma-rayenergy resolution in the CZT layer 12. In the exemplary embodimentdescribed here, as many as 12 pixels (worst case) out of 242 can beturned off and still meet a desired minimum efficiency, therebyincreasing the yield of usable CZT material.

In an exemplary embodiment, each CZT layer 12 may be formed of a 3 cm×3cm square chip, providing a total sensitive area of A=18 cm², separatedinto pixels 26. Each CZT/Si PIN assembly 24 may have an 11×11 array ofanode contacts such as is illustrated in FIG. 2, creating 121 pixels perchip (242 pixels total). This arrangement forms 242 pixels 26 in eachCZT/Si PIN assembly 24, and each pixel 26 in the Si PIN layer 14 isaligned with a pixel 26 in the CZT layer 12. Each pixel 26 independentlydetects neutrons and gamma rays. Individual defective pixels can beturned off via the controllers 30 and 32, significantly increasing theyield of usable CZT material, which is typically more expensive. Anexemplary pixel pitch is 2.6 mm, with a 0.7 mm setback at the outer edge(around the periphery).

As mentioned above, neutron detection is achieved through the gadoliniumconversion layer (Gd layer 20) on the glass plate 16. An exemplary Gdlayer 20 has a thickness of about 5 μm. For a 254,000 barn cross-sectionof ¹⁵⁷Gd (where a barn is defined as 10⁻²⁸ m²), the absorption rate inenriched elemental Gd is 7682 cm⁻¹. Thus a 5 μm Gd layer thickness willbe expected to capture 98% of all impinging thermal neutrons. Efficiencycan be increased by increasing the thickness of the Gd layer 20, butthat increased thickness comes with a corresponding increase in cost.

And as mentioned above, gamma-ray detection is achieved via the CZTlayer 12. For a thickness of d=0.35 cm, the CZT layer 12 has a totalvolume of 6.30 cm³. Using Harmonex software from Aprend Technology,available via www.aprendtech.com/OverView.html, the cross-section forphotoelectric interaction of a 1.17 MeV gamma ray in a 0.35 cm-thick CZTlayer 12 is about 0.002456 cm²/g. Given a density of 5.845 g/cm³, theabsorption rate in the CZT layer 12 is then about 0.01436 cm⁻¹. Althoughthis example uses a CZT layer 12 that has a thickness of 0.35 cm, thesensitivity to gamma rays can be significantly improved by increasingthe thickness to 1.0 cm, or 1.5 cm, or 2.0 cm, for example.

In optimizing the design of the detector, multiple parameters may beadjusted, including Gd layer 20 thickness, Si PIN layer 14characteristics, CZT layer 12 thickness, glass plate 16 thickness, biasvoltages, controller 30 and 32 operating conditions, and pixelcoincidence criteria. The exemplary example described here is but oneconfiguration.

Turning now to a description of the method of detecting neutrons andgamma rays using a common detector 10, where the detector includes alayer of semiconductor material (such as CZT) 12 adjacent a glass plate16 a layer of silicon PIN material 14 on an opposite side of the glassplate 16 and a Gd layer 20 between the glass plate 16 and the siliconPIN layer 14 to form a subassembly that is bounded by electrodes,including a semiconductor or first anode on one side of thesemiconductor material, a cathode connected to the glass plate, and a SiPIN or second anode on a side of the Si PIN layer opposite thesemiconductor anode. The method includes detecting electrons in the CZTlayer 12 or the Si PIN layer 14, and determining whether electrons weredetected in both the CZT and the Si PIN layers 12 and 14 at about thesame time, i.e. coincidentally. Consequently the method may be referredto as including a coincidence test. More particularly, the methodincludes the steps of (1) monitoring the electrical signal at each ofthe semiconductor anode and the Si PIN anode, and (2) comparing signalsfrom the semiconductor anode and the SI PIN anode to differentiatebetween gamma-ray events and neutron events based on predeterminedcriteria, described in further detail below. The method further includesestablishing an electric field within each of the CZT and the Si PINlayers 12 and 14 by generating a potential difference between the anodeand cathode contacts. The method also may include the step of providinga common gamma-ray and neutron detector with a controller connected tothe anodes and the cathode, the controller including a processor and amemory.

Through the photoelectric effect, a gamma ray deposits all of its energyin the CZT layer 12, generating a proportional number of electron-holepairs. In an applied electric field, the electrons and holes drifttoward their respective anode and cathode contacts, ideally generating asignal proportional to the gamma-ray energy. Because of trappingeffects, however, not all of the charge is collected. This problem hasbeen substantially overcome with the 3-D position-sensing technologythat has been implemented in the HPL2 ASIC, developed at BrookhavenNational Laboratory in Long Island, N.Y., US, making it an exemplarycontroller 30 and 32.

Each individual gamma event is corrected in amplitude according to thedepth in the CZT layer 12 at which the charge was generated. FIGS. 3Aand 3B show an example of how the 662 keV peak of ¹³⁷Cs has beensharpened from 2.2% to 0.72% full width at half maximum (FWHM) using the3-D technique mentioned above. An estimate of the width expected at 1.17MeV, scaling this by the square root of the ratio of the gamma rayenergies, leads to a linewidth of 0.54%. To be conservative, this can bedoubled to predict a linewidth of 1.15%. Most of the other gamma-rayevents will consist of Compton scattering, creating a broad background,especially if the ambient gamma field contains a variety of energies.Photoelectric gamma events, given reasonable statistics, will stand outabove this background because of their narrow lines.

Neutrons are detected by catching internal conversion electrons in theSi PIN layer 14. The neutrons are first captured in the Gd layer 20,producing the electrons that escape to the Si PIN layer 14. Neutroncapture by ¹⁵⁷Gd is represented by ¹⁵⁷Gd(n,γ+e⁻+x)¹⁵⁸Gd, in which thefinal nucleus, ¹⁵⁸Gd, relaxes to its ground state by releasing 7.9 MeVof energy in a complex mixture of gamma rays, internal conversion (IC)electrons, and x-rays. Typically more than three gamma rays emerge ineach neutron capture. The most probable energies occur at 79.51 and181.94 keV, with others between 0.606 and 6.75 MeV. All of these areunlikely to register in the Si PIN layer 14 because of the low atomicnumber of the Si PIN material and the relatively thin (typically 280 μm)thickness of the Si PIN layer 14. In comparison, prominent energies ofthe IC electrons are 79.51, 181.94, 255.67, and 277.55 keV, all of whichhave a small enough projected range to be fully stopped in the Si PINlayer 14. On average they will lose minimal energy emerging from the Gdlayer 20 before entering the Si PIN layer 14.

FIG. 4 is an actual spectrum of the IC electrons produced due toneutrons being captured in the Gd layer 20. (The angstrom notationsrepresent the kinetic energy of the thermal neutrons.) Energyspectroscopy of the IC electrons can be performed in the Si PIN layer14, in the same manner as gamma rays in the CZT layer 12, but withoutany need to correct for trapping. One electron-hole pair is created inthe Si PIN layer 14 for each 3.6 eV of IC electron energy, so each pulsewill be much greater than the dark current charge collected within pulseshaping time.

FIG. 5 illustrates the capture of a neutron in the detector 10. Theapproximate cathode-to-anode biases are 600V and 20V across the CZT andSi PIN layers 12 and 14, respectively.

The glass layer 16, generally at least 300 μm thick, functions: (1) tostand off the potential difference between the Si PIN and CZT cathodes,(2) to form a substrate for the Gd layer 20, (3) to stop IC electronsemitted by the Gd layer 20 from entering the CZT layer 12, and (4) toreduce the number of Compton-scattered electrons from the CZT layer 12that enter the Si PIN layer 14. Additionally, a thinner (5 μm) enriched,rather than a thicker natural Gd layer 20 is advantageous because itwill interact less with the gamma ray field and will permit easierescape of the IC electrons. The enriched Gd layer 20 also emits asimpler IC electron spectrum than the natural isotopic mixture.

For each event, the detector 10 provides six pieces of information: (1)pulse amplitude in the CZT layer 12 (if any); (2) pulse amplitude in theSi PIN layer 14 (if any); (3,4) timing of the CZT layer 12 and Si PINlayer 14 signals; and (5,6) the pixel locations in the CZT and Si PINarrays. The spectroscopy performed with the Si PIN layer 14 is not doneto reveal the neutron's kinetic energy, but to identify the ICelectrons. The manner in which these quantities are interpreted may beoptimized before assigning an event to a neutron or to a gamma ray withthe highest confidence. For example, a pulse occurring in the Si PINlayer 14 with no CZT layer 12 coincidence is most likely generated by aneutron, as long as the energy is compatible with the known IC electronspectrum. A coinciding event in the CZT layer 12, especially in a nearbypixel 26, assumed to represent a gamma ray event from the Gd layer 20,would confirm a neutron event. Because of timing, the amplitude could beignored, allowing either Compton scattering or photoelectric capture ofthe Gd-originated gamma ray in the CZT layer 12 to help confirm theneutron.

For gamma detection, a pulse in the CZT layer 12 with the absence of acoincident signal in the Si PIN layer 14, especially in neighboringpixels 26, would most likely indicate a gamma ray.

Accordingly, when the anodes are at zero volts, the glass plate 16stands off the potential difference between the two cathodes (negativevoltages indicated). Internal conversion electrons easily escape fromthe Gd layer 20 and deposit their energy in the Si PIN layer 14. Ifemitted in the other direction, the glass plate 16 prevents the ICelectrons from registering in the CZT layer 12. Gamma rays from the sameneutron event, if emitted downward, can be detected by the CZT layer 12.Because of the CZT-Si PIN coincidence test, both Compton scattering andphotoelectric absorption of gamma rays in the CZT layer 12 can be usedto help confirm the presence of a neutron event.

In summary then, the present invention provides a method for detectingboth gamma-ray events and neutron events with a common detector, wherethe detector includes a layer of semiconductor material adjacent oneside of a glass plate and a Gd layer on an opposite side of the glassplate, between the glass plate and a layer of silicon PIN material toform an assembly that is bounded by electrodes, including asemiconductor anode on one side of the semiconductor layer, a cathodeconnected to the glass plate, and a Si PIN anode on a side of the Si PINlayer opposite the semiconductor anode. The method includes the stepsof: (1) monitoring the electrical signal at each of the semiconductoranode and the Si PIN anode, and (2) comparing signals from thesemiconductor anode and the SI PIN anode to differentiate betweengamma-ray events and neutron events based on predetermined criteria.

Although the invention has been shown and described with respect to acertain preferred embodiment, it is obvious that equivalent alterationsand modifications will occur to others skilled in the art upon thereading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described components, the terms (including a reference to a“means”) used to describe such components are intended to correspond,unless otherwise indicated, to any component which performs thespecified function of the described component (i.e., that isfunctionally equivalent), even though not structurally equivalent to thedisclosed structure which performs the function in the hereinillustrated exemplary embodiments of the invention. In addition, while aparticular feature of the invention can have been disclosed with respectto only one of the several embodiments, such feature can be combinedwith one or more other features of the other embodiments as may bedesired and advantageous for any given or particular application.

1. A detector for both gamma-rays and neutrons, comprising: asemiconductor layer including a semiconductor material suitable forcapturing gamma-rays; a glass plate in contact with the semiconductorlayer; a gadolonium (Gd) converter layer in contact with the glass plateopposite the semiconductor layer for capturing neutrons; a layer ofsilicon PIN (p-type/intrinsic/n-type) material of suitable thickness incontact with the Gd converter layer opposite the glass plate to detectelectrons produced by neutrons captured in the Gd converter layer; acathode contact in electrical contact with the glass plate; a firstanode contact in contact with the semiconductor layer; a second anodecontact in contact with the silicon PIN layer; and a processor inelectric contact with the first and second anode contacts, the processorbeing configured to cooperate with the anode contacts and the cathodecontacts to establish electric fields across the semiconductor layer andthe Si PIN layer, and being configured to differentiate between signalsgenerated by a neutron event and signals generated by a gamma-ray event.2. A detector as set forth in claim 1, where the semiconductor materialincludes any of cadmium-zinc-telluride (CdZnTe), high-resistivitygallium arsenide (GaAs), and high purity germanium (HPGe).
 3. A detectoras set forth in claim 1, further comprising a printed circuit board(PCB) in electrical contact with respective ones of the first and secondanode contacts to connect the first and second anode contacts torespective contacts in respective processors.
 4. A plurality of adjacentspaced-apart detectors as set forth in claim 3, with a plurality ofdetectors sharing common PCBs.
 5. A detector as set forth in claim 1,where the first and second anode contacts are divided into an array ofpixels.
 6. An array of detectors as set forth in claim
 1. 7. A detectoras set forth in claim 1, where the semiconductor layer includescadmium-zinc-telluride (CdZnTe) and is separated into pixels alignedwith pixels in the Si PIN layer.
 8. A detector as set forth in claim 1,where the semiconductor layer has a thickness of 0.35 cm, the glassplate has a thickness of at least 300 μm, the Gd layer has a thicknessof 5 μm, and the Si PIN layer has a thickness of 280 μm.
 9. A detectoras set forth in claim 1, where the semiconductor layer has a thicknessof 1.0 cm or 1.5 cm or 2.0 cm.
 10. A controller for use with a commondetector for both gamma-rays and neutrons including a semiconductorlayer including a semiconductor material suitable for capturinggamma-rays, a glass plate in contact with the semiconductor layer, agadolonium (Gd) converter layer in contact with the glass plate oppositethe semiconductor layer for capturing neutrons, a layer of silicon PIN(p-type/intrinsic/n-type) material of suitable thickness in contact withthe Gd converter layer opposite the glass plate for detecting electronsproduced from a neutron captured in the Gd converter layer, a cathodecontact in electrical contact with the glass plate, a first anodecontact in contact with the semiconductor layer, and a second anodecontact in contact with the silicon PIN layer, the controllercomprising: a processor configured to cooperate with the first anode,the second anode and the cathode to establish an electric field acrossthe semiconductor layer and the silicon PIN layer; and the processor isconfigured to differentiate between signals generated by a neutron eventand signals generated by a gamma-ray event as a function of coincidencetesting of signals received from each of the first and second anodes.11. A method for detecting both gamma-ray events and neutron events witha common detector, the detector including a layer of semiconductormaterial adjacent a glass plate, a gadolinium (Gd) converter layeradjacent an opposite side of the glass plate, and a layer of silicon PINmaterial in contact with the Gd converter layer on an opposite side ofthe glass plate to form a subassembly that is bounded by electrodes,including a semiconductor anode on one side of the semiconductor layer,a cathode connected to the glass plate, and a Si PIN cathode on a sideof the Si PIN layer opposite the semiconductor anode, the methodcomprising the steps of: monitoring the electrical signal at each of thesemiconductor anode and the Si PIN cathode; and comparing signals fromthe semiconductor anode and the Si PIN cathode to differentiate betweengamma-ray events and neutron events based on predetermined criteria. 12.A method as set forth in claim 11, comprising the step of establishingan electric field within the semiconductor layer and the Si PIN layer.13. A method as set forth in claim 11, including the step of providing acommon gamma-ray and neutron detector with a controller connected to thefirst and second anodes and the cathode, the controller including aprocessor and a memory.
 14. A detector for both gamma rays and neutrons,comprising: means for capturing gamma-rays that produces an electron;means for capturing neutrons that produces an electron; means forseparating electrons generated by the means for capturing gamma raysfrom the means from electrons generated by the means for capturingneutrons; means for establishing an electric field across the means forcapturing neutrons and the means for capturing gamma rays; and means fordifferentiating between signals generated by a neutron event and signalsgenerated by a gamma-ray event.
 15. A detector as set forth in claim 14,where the means for capturing gamma-rays includes a semiconductor layerhaving a semiconductor material suitable for capturing gamma-rays.
 16. Adetector as set forth in claim 15, where the means for capturinggamma-rays includes a layer of crystallized cadmium-zinc-telluride(CdZnTe) (CZT).
 17. A detector as set forth in claim 14, where the meansfor capturing neutrons that produces an electron includes a gadolonium(Gd) converter layer for capturing neutrons and producing an electronand a layer of silicon PIN (p-type/intrinsic/n-type) material ofsuitable thickness in contact with the Gd converter layer for detectingthe electron.
 18. A detector as set forth in claim 14, where the meansfor separating electrons generated by the means for capturing gamma raysfrom electrons generated by the means for capturing neutrons includes aglass plate.
 19. A detector as set forth in claim 14, where the meansfor applying an electric field across the means for capturing neutronsand the means for capturing gamma rays includes: a cathode contact inelectrical contact with the separating means; a first anode contact incontact with the gamma-ray capturing means; a second anode contact incontact with the neutron capturing means; and a processor in electriccontact with the first and second anode contacts, the processor beingconfigured to cooperate with the first and second anode contacts and thecathode contacts to establish an electric field across the means forcapturing neutrons and the means for capturing gamma rays.
 20. Adetector as set forth in claim 14, where the means for differentiatingbetween signals generated by a neutron event and signals generated by agamma-ray event includes a processor in electric contact with the meansfor establishing an electric field across the means for capturingneutrons and the means for capturing gamma rays, the processor beingconfigured to differentiate between signals generated by a neutron eventand signals generated by a gamma-ray event.